Naveen Santhapuri, Srihari Nelakuditiand Romit Roy Choudhury

University of South Carolina Duke University

WCNC 2008

OutlineIntroductionRelated workCASTSimulation Conclusions

Introduction The optimal network capacity for wireless

networksRelated to spatial reuse

Efficient spatial reuse is inhibited by InterferenceExternal noiseOther physical factorsLimitations of MAC protocols

Introduction The 802.11 protocol with its virtual carrier

sensing has role reversals which Reduce the hidden node problem Introduce the exposed node problem

Introduction If there was no ACK

Two nodes can transmit DATA simultaneouslyExposed sender problem can be solved

partially

Piggybacked ACK mechanism“Piggybacked-Ack-aided Concurrent

Transmissions”N. Santhapuri, J. Wang, Z. Zhong, and S.

NelakuditiICNP Poster Session, 2005

A B CDATART

S

CTS for AACK for C

Introduction -- collision Multiple packets arriving at a receiver are

generally considered to cause packet loss due to the collision at the receiver

A B C

Introduction A sufficiently stronger frame can still be

successfully received by the receiver in spite of a collisionThis phenomenon is called physical layer

capture (PLC)

Related work“Sniffing out the correct physical layer

capture model in 802.11b” A. Kochut, A. Vasan, A. U. Shankar and A.

AgrawalaICNP, Oct. 2004

R1 S1 S2 R2

ICNP-2004

S1R1S3

R1 S1 S2 R2 S3

S1R1S2

Preamble

DATA

X X

100m50

m50m

50m

Related work“An experimental study on the capture effect

in 802.11a networks”J. Lee, W. Kim, S.-J. Lee, D. Jo, J. Ryu, T. Kwon

and Y. ChoiWinTECH, Sept. 2007

WinTECH-2007

A signal is significantly more vulnerable to interference if it starts after the interfering frame than had it started before the interfering frame.

S1R1S3

S1R1S2

interest

interferingX

interest

interferingX

Preamble

DATA

Proposed approachAssumption

Assume two hop signal strength informationS1’s signal strength at R1 will be known by S2

R1 S1 S2100m50

m

Proposed approachCapture-Aware Staggering of Transmissions

(CAST)

S1 ->R1

RTS

DATA

R1 S1 S2 R2 S3100m50

m50m

50m

802.11

CASTS1 ->R1S2 ->R2

ACK

CTS

PWAIT

CASTRTS: S1 -> R1

No other frame > SF hold

S1 ->R1

RTS

DATA

802.11

CASTS1 ->R1

S2 ->R2

ACK

CTS

PWAIT

The SINR value at R1 for a signal from S1in the presence of interference from S2

The SINR value at R1 for a signal from S1in the presence of interference from S2

2

11

2

11 , R

RSSRS SS

CASTCTS: R1 -> S1RTS: S2 -> R2 after one physical preamble

CTS startsMedium is free > SF hold

RTS starts > SLC hold

S1 ->R1

RTS

DATA

802.11

CASTS1 ->R1

S2 ->R2

ACK

CTS

PWAIT

1

22

RRSS

2

11

2

11 , R

SRSSR SS

CASTCTS: R2 -> S2DATA: S1 -> R1 after 2 physical preamble

times

CTS startsMedium is free > SF hold

DATA starts > SLC hold

S1 ->R1

RTS

DATA

802.11

CASTS1 ->R1

S2 ->R2

ACK

CTS

PWAIT

1

22

1

22 , R

SRSSR SS

2

11

RRSS

CASTDATA: S1 -> R1DATA: S2 -> R2

S1 -> R1Medium is free > SF hold

S2 -> R2 > SLC hold

S1 ->R1

RTS

DATA

802.11

CASTS1 ->R1

S2 ->R2

ACK

CTS

PWAIT

2

11

2

11 , R

RSSRS SS

1

22

SRSS

CASTIn 802.11, for two concurrent transmissions

8 signal strength value must be above SLC threshold

In CAST, for two concurrent transmissionsOnly 5 signal strength value must greater than

SLC

1

22

1

22

2

11

1

22

2

11 , , , , R

RSSRS

RRS

SSR

SRS SSSSS

Optimize the protocol

RTS

DATA

CASTPrimary

Secondary

CTS

PWAIT

CASTPrimary

Secondary

SLC or SLC 1

22

2

11 S

RSSRS SS

Simulation Qualnet simulatorTraffic flow: randomTraffic rate: CBRPacket size: 512 Bytes

Grids in a fixed sized areaSpace: 1000m*1000mEach grid has a different grid unit

75m ~ 175m

Random TopologiesSpace: 1000m*1000mNodes: 100Randomly generated 40 1-hop flows

Conclusions Spatial reuse in wireless networks is limited

by the SINR threshold requirementsThis problem is amplified because of role

reversals in wireless networksPhysical layer capture can improve the spatial

reuse by staggering the transmissionsSimulation results show that the number of

concurrent transmissions can be improved significantly

Thank you